Agricultural and Forest Meteorology 142 (2007) 96–102
www.elsevier.com/locate/agrformet
Agriculture’s influence on climate during the Holocene
M. James Salinger *
National Institute of Water and Atmospheric Research, P.O. Box 109 695, Auckland, New Zealand
Received 25 November 2004; received in revised form 15 November 2005; accepted 15 March 2006
Abstract
This paper summarizes the variations of trace gas behaviour and climate during the Holocene (approximately the last 10,000 years),
with reference to the last four ice age cycles. The industrial era, commonly regarded as commencing during the 18th century, is one
noted when atmospheric greenhouse gas increases due to burning of fossil fuels and land use changes have been attributed to increases
in global average near-surface temperatures, particularly in the latter part of the 20th century. However, analysis by Ruddiman has noted
that in the Holocene during the period of civil society, the changes in atmospheric greenhouse gases have been anomalous compared
with the geological record of the last 400,000 years. During this period, both carbon dioxide (CO2) and methane (CH4) increased,
probably as a result of the introduction of agrarian agriculture and land clearing in Eurasia. These, and other land use changes because of
agrarian rural activities, may have caused a subtle forcing of climate, preventing climate cooling which might have been expected
because of natural forcing. If future evidence supports the Ruddiman hypothesis, then agricultural and forestry activities during the
period of civil society may have been exerting an influence on climate for, at least, the last 8000 years.
# 2006 Elsevier B.V. All rights reserved.
Keywords: Climate; Climate change; Agriculture; Atmospheric greenhouse gases; Holocene; Industrial era
1. Introduction
On the background of internal climate variability,
external mechanisms such as volcanism and the increase
of greenhouse gases from anthropogenic activities have
acted (Salinger et al., 2000) in the industrial era.
Modelling studies have identified the external factors in
the period of current climate. From model simulations,
IPCC (2001) concluded that climate forcing from
changes in solar radiation and volcanism is likely to
have caused fluctuations in global and hemispheric mean
temperatures in the first part of the 20th century.
However, these have been too small to produce the mean
temperature increases in the latter part of the 20th
century. Well mixed greenhouse gases (carbon dioxide
(CO2), methane (CH4), chlorofluorcarbons, etc.) have
* Fax: +64 9 375 2051.
E-mail address: [email protected].
0168-1923/$ – see front matter # 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.agrformet.2006.03.024
made the largest contribution in radiative forcing to warm
the climate in the late 20th century, as now validated by
climate model simulations of global-average surface
temperature. It is the growth in these greenhouse gases
that have caused the climate warming during the
industrial era, the start of which the IPCC has placed
around 1750 A.D.
Past studies (IPCC, 2001) have shown a strong
linkage between fluctuations in CO2 and CH4 with
temperature during the last four glacial cycles.
However, Ruddiman (2003) has advanced ideas that
greenhouse forcing of climate due to human activities
may have began shortly after agrarian civilisation
developed during the Holocene. This contribution will
summarise the trends between atmospheric greenhouse
gases and climate forcing, as depicted by changes in
average surface temperature during the industrial era,
before tracing the course of CO2 and CH4 and
temperature over the last four glacial/interglacial
M.J. Salinger / Agricultural and Forest Meteorology 142 (2007) 96–102
cycles. The discontinuity in trends between CO2 and
CH4 and climate factors during the Holocene, as noted
by Ruddiman (2003) will be traced during the Holocene
and the concept of early forcing of climate by human
agricultural and land-use activities examined.
2. The industrial era
The industrial era is commonly regarded as that
period of time, from about the Industrial Revolution
onwards, with the invention of the steam engine and
the growth of consumption of fossil fuels as a source
97
of energy for society. Crutzen and Stoermer (2000)
have placed the commencement of this period at
1800 A.D., but here the start will be placed at
1750 A.D., commensurate with the start of increases
in the well mixed greenhouse gases noted by the IPCC
(2001).
Since then, human activities through the burning of
fossil fuels, deforestation, and land-use changes have
led to increases in greenhouse gases (Fig. 1). During the
industrial era, CO2 concentrations have increased from
about 280 ppm in the pre-industrial era to 376 ppm in
2004 (Keeling and Whorf, 2004), a 34% increase. In the
Fig. 1. Records of changes in atmospheric composition of carbon dioxide, methane and nitrous oxide over the past thousand years (a). Sulphate
concentrations from Greenland ice cores are shown in (b) (Source: IPCC, 2001).
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M.J. Salinger / Agricultural and Forest Meteorology 142 (2007) 96–102
past 20 years, about three-quarters of these emissions
have been put down to fossil fuel burning, and the
remainder to land-use changes (IPCC, 2001). Atmospheric CH4 has increased from about 700 to 1760 ppb
by 2000 (IPCC, 2001), a 151% increase. Slightly more
than half of the recent emissions are from the use of
fossil fuels, livestock emissions, rice agriculture and
landfills. The growth in these and other greenhouse
gases during the industrial era over the 250 year period
has been estimated to contribute warming equivalent
to an extra 2.43 W m 2 to the climate system, which
compares with a slight cooling of
0.4 W m 2
produced by emissions of aerosols.
Global mean surface temperatures have increased by
0.6 8C over the 20th century. Fig. 2 shows that recent
years have been among the warmest in the period of
instrumental temperature records. The 10 warmest
years all occurred in the 1980s and 1990s. Many are in
the 1990s with 1998 being the warmest, followed by
2002 then 2003.The recent warming in the northern
hemisphere, at least, appears to be the warmest for the
last millennium. The warming over the past century
began during one of the colder periods. The published
material suggests that the 1990s, for the northern
hemisphere at least, are the warmest that they have been
for the entire millennium. Information from ice cores
indicates that the sharp 20th century warming appears to
have been preceded by irregular cooling, and is
therefore more striking.
The comparison of global mean surface temperature
anomalies relative to the 1880–1920 instrumental
record compared with ensembles of four simulations
with a coupled ocean–atmosphere climate model (Stott
et al., 2000; Tett et al., 2000; IPCC, 2001) accounts for
Fig. 2. Global surface temperatures (a) over the period 1860–1999
from instrumental temperature observations and (b) Northern Hemisphere surface temperature trends over the last millennium from
indicators of past climate (Source: IPCC, 2001, updated from UK
Meteorological Office, Hadley Centre).
the causal factors behind these temperature trends.
From the simulations, IPCC (2001) concluded that
climate forcing the changes in solar radiation and
volcanism is likely to have caused fluctuations in global
and hemispheric mean temperatures in the first part of
the 20th century. However, these have been too small to
produce the mean temperature increase in the latter part
of the 20th century. Well mixed greenhouse gases (CO2,
CH4, chlorofluorcarbons, etc.) must have made the
largest contribution in radiative forcing to warm the
climate in the late 20th century, as now validated by the
above mentioned climate model simulations of global
average surface temperature.
However, because of the rapidness of increase in
greenhouse gases in the industrial era (Fig. 1) compared
with the last millennium, with about half the increases
in CO2 and CH4 in the latter half of the 20th century,
there is a thermal inertia in the climatic system to
warming because of the long thermal response time of
the ocean, estimated to be of several decades (Hansen
et al., 1984). Therefore, there is still a global warming
commitment to occur once the climatic system comes
into equilibrium with the increased atmospheric greenhouse gas concentrations.
3. The last four glacial cycles
The last four glacial cycles/interglacial cycles
(Fig. 3) over the last 420,000 years show coupling
between changes in solar radiation, CO2, CH4 and
temperature. The solar radiation changes are closely
linked to orbital factors (Berger and Loutre, 1996). For
CH4, a highly coherent match has been noted with the
23,000-year orbital insolation cycle (Ruddiman and
Raymo, 2003). This supports the hypothesis that
strengthening of the tropical monsoons (Brook et al.,
1996) during the northern hemisphere summer insolation peak increases the vigour of the monsoon
precipitation (Kutzbach, 1981), which may increase
monsoon wetlands, releasing CH4 to the atmosphere
(Ruddiman, 2003).
The much more abundant greenhouse gas, CO2,
shows variations on all three orbital periods, with the
100,000-year cycle dominant (Petit et al., 1999).
Ruddiman notes that the phase of the 23,000-year
CO2 signal lags northern hemisphere summer insolation
by <1000 years, the 41,000-year CO2 signal by an
average of 6500 years, and is nearly in phase with
insolation for the dominant 100,000-year cycle (Raymo,
1997). Thus, atmospheric CO2 concentrations show
good coupling with the orbital cycles, especially the
dominant 100,000-year period of eccentricity.
M.J. Salinger / Agricultural and Forest Meteorology 142 (2007) 96–102
99
Fig. 3. Changes in solar radiation, temperature (K), atmospheric concentrations of methane and carbon dioxide measured at the Vostok Ice core over
the last 420,000 years, which includes four glacial/interglacial cycles (from Petit et al., 1999).
In summary, all the evidence points to tight coupling
between variations in northern hemisphere summer
insolation, CO2, CH4 and temperature from ice core
data. The boreal summer insolation peaks, because of
the orbital cycles, are reasonably coincident with higher
concentrations of CO2, CH4, as are the boreal summer
insolation minima with lower atmospheric CO2 and
CH4 concentrations. Atmospheric CO2 concentrations
have ranged over the majority of the last four glacial/
interglacial cycles between about 180 and 280 ppm, and
CH4 concentrations 450–700 ppb. Fig. 3 shows that
temperature variability has been reasonably synchronized with the CO2 and CH4 fluctuations, with glacial
periods occurring when these trace gas concentrations
are lower, and interglacial periods when trace gas
concentrations are higher. Ice core evidence (Fig. 3)
suggests that the difference in surface temperatures
between these two states of the climatic system were in
the order of 5 8C. In summary, climate and trace gas
variability prior to the Holocene period (the last 10,000
years) has been clearly driven by natural factors.
three interglacials. Values then decreased to about
260 ppm by 8000 years ago, before beginning an
anomalous increase to 280–285 ppm by 2000 years ago
(Fig. 4). This increase is clearly anomalous because the
last eccentricity maximum occurred about 13,000 years
ago, coincidently with a CO2 maximum near that time.
This should have been followed by a long-term decrease
as northern hemisphere summer insolation reduced
(Fig. 3).
Though for the present Holocene interglacial, the
early decrease follows the first peak, and then CO2
4. The Holocene
Ruddiman (2003) posits that the Holocene trends in
CO2 and CH4 are different than that might be expected
from natural forcing factors. High resolution precision
records from Taylor Dome in Antarctica (Indermuhle
et al., 1999) show that CO2 reached a peak of 268 ppm
between 11,000 and 10,000 years ago, which shows the
same relative placement as the CO2 maxima in the past
Fig. 4. High-resolution of CO2 of Indermuhle et al. (1999) from
Taylor Dome, Antarctica for the Holocene. The Holocene CO2 trend
has been projected toward values reached during the previous interglaciations (Source: Ruddiman, 2003).
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M.J. Salinger / Agricultural and Forest Meteorology 142 (2007) 96–102
Fig. 5. The GRIP CH4 record from Blunier et al. (1995). The Holocene CH4 trends projected are based on the previous interglacial CH4 trends with
the 23,000-year orbital cycle (Source: Ruddiman, 2003).
concentrations in the atmosphere climb. During this
period, Ruddiman (2003) notes the rise of land clearing
and commencement of agriculture with civilized
society, a situation quite different than for the other
three interglacials. The initial spread of agriculture and
deforestation, in the Fertile Crescent and eastern
Mediterranean, has been placed at 8–10,000 years
ago by Zohary and Hopf (1993), with an advance
westward into other parts of Europe by about 6000 years
ago. Robeets (1998) has mapped ‘stratified’ agriculture
and simple ‘peasant’ agriculture as at 2000 years ago,
and shows extensive areas of Eurasia, South America,
and smaller parts of North America supporting this type
of activity. An estimate of 224–245 gigatonnes of
terrestrial carbon release due to deforestation with the
introduction of agriculture by 2000 years ago (Ruddiman, 2003) is enough to give the 20–25 ppm CO2 raise
from 8000 to 2000 years ago. Prior to 1500 A.D. there
was also the spread of grassland and cultivation of
crops. The subsequent fluctuations from 2000 years ago
until 1970 A.D. are put down to farm abandonment
causing declines in atmospheric CO2, and rural revivals
causing increases.
High resolution CH4 records from Greenland ice
(Fig. 5) shows the most recent maximum, reaching about
700 ppb just prior to 10,000 years ago (Blunier et al.,
1995), coincidently, the peak summer insolation values in
the northern subtropics, with values declining to about
600 ppb at 5000 years ago. Values then trend anomalously upwards to 700 ppb by 1970 A.D. This rise is not
consistent with the orbital-monsoon hypothesis that has
persisted for the last 350,000 years, which would suggest
a decline to atmospheric CH4 concentrations of about
450 ppb by the start of the industrial era.
It has been proposed that the anomalous rise in CH4
from 5000 years ago by about 250 ppb (Ruddiman and
Thomson, 2001) is explained by anthropogenic factors
such as the commencement and growth of irrigated rice
agriculture (Robeets, 1998) in China and other parts of
South Asia, coupled with other innovations in
agriculture. During this same period, there has been a
growth in animal husbandry leading to ruminant
livestock grazing, associated with the spread of
grassland prior to 1500 A.D. Both these types of
agriculture produce emissions of atmospheric CH4.
These developments in agriculture are in the order of
M.J. Salinger / Agricultural and Forest Meteorology 142 (2007) 96–102
magnitude sufficient to explain the 250 ppb rise in CH4,
compared with the likely trend, by 1750 A.D.
101
The previous sections indicate that prior to the
industrial era, many of the changes leading to the likely
anomalous increases of CO2 and CH4 are probably a
result of civilised society introducing agrarian activities.
From 8000 years ago, these societies cleared forests in
many areas of Eurasia, Africa and the Americas, and
introduced both peasant and more complex agricultural
systems. More latterly, the development of rice
agriculture, spread of cropping, cultivation of grasslands
and associated animal husbandry would have contributed
to these trends in CO2 and CH4. The anomalous growth in
these greenhouse gases during the late Holocene,
compared with previous interglacials prior to the
industrial era, would produce impacts on temperature.
Using an IPCC (2001) estimate of 2.5 8C equilibrium
global climate sensitivity to CO2 and CH4 doubling the
anomalous 40 ppm CO2 and 250 ppb CH4, translates to
a global warming of about 0.8 8C by 1750 AD. During
the period from 8000 years ago to 1750 A.D., because
of the slow rise in CO2 and CH4 the atmosphere and
oceans of the climate system would have time to come
into thermal equilibrium to the slightly increased levels
of these greenhouse gases. Ruddiman (2003) notes that
this warming trend may have been masked by the
declining summer northern hemisphere insolation
levels as a result of the obliquity and precession orbital
cycles (Kutzbach et al., 1996) in the late Holocene.
Conversely, during the industrial era from
1750 A.D., the rapid increases in greenhouse gases
have only started to be expressed in global temperature
trends because of the thermal inertia of the oceans,
estimated at several decades (Hansen et al., 1984) for
climate to come into equilibrium with current atmospheric concentrations of CO2 and CH4.
biomass activity in the southern hemisphere, which is
80% oceanic, with orbital insolation changes is expected
to be slight. It appears then that the northern hemisphere
has been a driver in trace gas changes and glacial
initiation and deglaciation.
However, the recent anomalous increases in CO2
since about 8000 years ago, and CH4 from 5000 years
ago to the commencement of the industrial era around
1750 A.D. is at variance with the orbital cycle. New
factors have to be evoked to explain the increases
against the expected trend. These pre-industrial
increases are likely to be caused by the spread of early
agriculture with forest clearance from 8000 years ago,
introduction of cereal cropping, the growth of paddy
rice cultivation and the spread of grasslands with the
introduction of livestock husbandry. These factors
suggest that agriculture was a major influence causing
the increases in CO2 and CH4 against the expected trend
from 8000 years ago to about 1750 A.D., and thus
preventing the cooling of climate and the first stages in
perhaps the initiation of northern high latitude ice sheet
development. During the industrial era, agriculture has
contributed to CO2 and CH4, as well as nitrous oxide.
During the last 20 years of the 20th century, land-use
changes contributed to between 10 and 30% of CO2
emissions (IPCC, 2001). About half of CH4 and nitrous
oxide emissions are from agricultural activities. These
have all contributed to some of the warming observed in
the latter part of the 20th century. However, because of
lags in the climatic system caused by the thermal inertia
of the oceans, the full climate warming of the
agricultural contribution during the industrial era has
yet to be expressed. In contrast, because the rise of
atmospheric CO2 and CH4 concentrations caused by
agricultural activities was slow prior to about
1750 A.D., then the climatic system had time to fully
equilibrate to the changed levels of these trace gases. It
can be concluded that agriculture, very likely subtly but
significantly, influenced climate during the Holocene.
6. Conclusions
Acknowledgement
Variations in CO2 and CH4 over the last four glacial/
interglacial cycles prior to the Holocene are likely to have
been driven by Earth-orbital changes. These cycles of
eccentricity, obliqueness and precession, with periods of
100,000, 41,000 and 23,000 years increase and decrease
summer insolation received in the northern hemisphere,
which are a key to driving monsoonal vigour and biomass
activity in the terrestrial hemisphere, with resulting
changes in atmospheric concentrations of CO2 and CH4.
In comparison, with the influence of such changes on
This research was support by the New Zealand
Foundation for Research, Science and Technology
under contract CO1X0202.
5. Holocene agriculture and climate links
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